[Web4lib] 'request timed out' versus 'destination host unreachable'

[John Fitzgibbon]

Hi,
 
I'm preparing a short document on network troubleshooting for the staff
here. When a server is down, we sometimes get a 'destination host
unreachable' message from a router when we ping it. Sometimes, we get
the message 'request timed out'. My guess is that the latter is caused
by the nearest router being too busy to reply. Am I right in this?
 
Also, when a router fails to get a reply (request timed out) from the
server that is down does it send a 'destination host unreachable'
message back up the line? What precisely is the difference between the
two responses that we get?
 
Also, I'm unable to simulate a 'destination network unreachable' message
even when I type in a nonsensical IP address that is definitely not on
our network.
 
Any enlightenment would be much appreciated.
 
Regards
John

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